In recent years, light sources such as light emitting diodes (LEDs) have been improved through technological advances in material and manufacturing processes. The LEDs possess characteristics such as a relatively high efficiency, a relatively long life, and vivid colors, and can be used in a variety of industries. One example is to use the LEDs to replace traditional incandescent bulbs in a vehicle lamp. Compared with traditional incandescent bulbs, the LEDs are lighter, compact, long-life, and energy-saving. Moreover, the response time of the LEDs is faster than that of the incandescent bulbs.
For some vehicles that are originally designed to be equipped with incandescent bulbs, there will be a problem if the incandescent bulbs are directly replaced by LEDs.
FIG. 1 illustrates a conventional circuit for using an incandescent bulb in a vehicle. The incandescent bulb 102 is powered by a power source 100, e.g., a battery, via a power line. Under certain circumstances, a vehicle may need to perform a self-testing to examine whether the incandescent bulb 102 is turned on properly. As the vehicle starts, a micro controlling unit (not shown in FIG. 1) in the vehicle may generate a testing signal (usually a square wave signal) and apply the testing signal to the power line. A detecting circuit (not shown in FIG. 1) monitors the voltage drop across the incandescent bulb 102. If a waveform of the testing signal has an amplitude greater than a predetermined level, the waveform can be detected by the detecting circuit. If the incandescent bulb 102 operates properly, the voltage drop (or current) across the incandescent bulb 102 is relatively small because the resistance of the filament in the incandescent bulb 102 is relatively small. That is, in operating properly, a constant current is created in the whole loop, and a current sensor 103 will sense the correct current value. Therefore, the waveform of the testing signal is not detected by the detecting circuit. If the incandescent bulb 102 is broken down, the current is relatively small or even causing an open circuit condition; then, the waveform of the testing signal can be detected by the detecting circuit across the incandescent bulb 102. If the testing signal is detected by the detecting circuit, the detecting circuit can determine that the incandescent bulb 102 is broken and remind the driver by turning on an indicator light on the dashboard. It is followed by maintaining or repairing the vehicle. Moreover, a switch 101 is connected in series to the incandescent bulb 102 for switching. The current sensor 103 is coupled to the incandescent bulb 102.
FIG. 2 illustrates a conventional circuit using LEDs to replace a traditional incandescent bulb in a vehicle. As shown in FIG. 2, a LED string 104 replaces the incandescent bulb. The LED string 104 includes multiple LEDs connected in series. Generally, the resistance of the LED string 104 is greater than the resistance of an incandescent bulb. Similarly, a switch 101 is connected in series to the LED string 104 for switching. The current sensor 103 is coupled to the LED string 104. As the incandescent bulb replaced by the LED string, the LED's power consumption is relatively small such that the loop current becomes small. If the current detected by a circuit system is lower than normal value, it will result in a warning to notify the driver.
FIG. 3 illustrates another conventional circuit for a vehicle lamp. In an example, the LED string 104 is connected to a dummy load 105 in parallel. As above-mentioned, the resistance of the LED string 104 is greater than that of an incandescent bulb. Therefore, when the micro controlling unit (not shown in FIG. 3) applies the testing signal on the power line, a waveform of the testing signal may be detected by the detecting circuit (not shown in FIG. 3) across the LED string 104, even if the LED string 104 operates properly. The micro controlling unit may render an erred judgment. To prevent a false alarm, a dummy load 105 is coupled to the LED string 104 in parallel. In order to generate an enough current, the both ends of the LED string 104 may be connected to a high-power electronic component as the dummy load 105 in parallel. In an example, the dummy load 105 is a resistor. The resistor 105 can have a relatively small resistance such that the total resistance of the parallel-connected dummy load 105 and the LED string 104 is even smaller. By properly choosing the resistance of the resistor 105, the testing signal is not detected by the detecting circuit across the LED string 104 such that the false alarm can be avoided. A drawback of this solution is that the resistor 105 will constantly consume power and generate heat if the vehicle lamp is turned on. Besides, a switch 101 is connected in series to the LED string 104 and the dummy load 105 for switching. The current sensor 103 is coupled to the LED string 104 and the dummy load 105. In one example, the current path includes a dummy load 101 and a switch 101 coupled in series.
FIG. 4 illustrates yet another conventional circuit for a vehicle lamp. Similarly, the LED string 121 is connected to a dummy load 131 in parallel. A switch 111 is connected in series to the combination of the LED string 121 and the dummy load 131 for switching. And, switches 112, 113 and 114 are connected in series to the combinations of LED strings 122, 123, 124 and dummy loads 131, 132, 133 for switching, respectively. In this example, the whole circuit includes four set combinations of the LED strings and the dummy loads. The current sensor 103 is coupled to the four set combinations of the LED strings and the dummy loads. A drawback of this solution is that the combinations of the LED strings and the dummy loads will constantly consume power and generate heat if the vehicle lamp is turned on. The complete set of the vehicle lamps may represent as four or five functions, and it needs to use the multiple combinations of the LEDs and the dummy loads such that the generating heat is a relatively large, and the volume becomes large. Besides, the current sensor 103 is coupled to the multiple combinations of the LED strings and the dummy loads.
Therefore, based-on the shortcomings of prior arts, the present invention provide a newly circuit for driving a vehicle lamp.